This invention relates to chemically hardened optical crown glass.
Both the thermal and the chemical hardening of glasses are known. For increasing the tensile and impact strength of optical quality crown glasses used as eyeglasses, however, only thermal hardening processes have heretofore been used, because conventional eyeglasses due to their chemical composition were not suited to be chemically strengthened to such an extent as to achieve satisfactory results in drop ball testing.
The thermal hardening does however involve disadvantages. This hardening process is unsuitable for a glass blank and has to be carried out on a glass which has already been ground, polished and rimmed. For thermal hardening, it is necessary to heat the glasses to high temperatures and then to quench them. This heating passes into transformation temperature ranges about 550.degree. C. in which the glass is already deformed after several minutes. As a result, there is the danger that the surface curves which are carefully produced and often are specially calculated to correct for visual errors become distorted.
Another disadvantage of the thermal hardening is the parabolic distribution of tension which is thereby achieved in the cross-section of a thermally hardened optical glass lens. This is always noticeable in a displeasing manner when the corrective glass lenses have a non-uniform thickness. With the hardening, a stress imbalance is produced so that it is difficult to accurately determine what stressing the thermally hardened glass will actually withstand in subsequent use.
An additional disadvantage of thermal hardening is that it is restricted to lenses having a minimum thickness of about 2 mm. This means that a heavier spectacle glass is often required, particularly with corrective glasses having a negative diopter since it is necessary to have a minimum central thickness in the thinnest part of the glass.